A conventionally known depolarizer converts completely polarized or partially polarized input light into non-polarized light before output (see, e.g., Japanese Laid-Open Patent Publication Nos. H7-248422 and H7-159632). The depolarizer is implemented by a long polarization maintaining fiber (PMF), for example. A Lyot-type depolarizer is known that is fabricated by fusing two PMFs at 45 degrees. The depolarizer is used for converting excitation light into non-polarized light in a Raman amplifier, which amplifies light by utilizing the spontaneous Raman scattering of optical fibers.
Nonetheless, the conventional techniques have a problem of increased apparatus size consequent to the long polarization maintaining fiber.